1. Field of the Invention
The present invention relates to a battery voltage measuring system for measuring voltage of a battery mounted in a portable device. This Patent Application is based on Japanese Patent Application No. 2007-004274. The disclosure of the Japanese Patent Application is incorporated herein by reference.
2. Description of Related Art
Portable devices driven by cells or batteries have been known. The portable devices are such as a notebook-sized PC (Personal Computer), a mobile phone, a PDA (Personal Digital Assistant), a digital camera, a digital video (digital video camera) and a portable audio device. In recent years, capabilities of the portable devices have been dramatically improved by accompanying downsizing and advancement of semiconductor devices built therein. In conjunction with this, a control of voltage of the battery built in the portable device (battery voltage) has become important to operate the portable devices stably. The precise battery voltage needs to drive the portable device with the battery for a long time. For this reason, a battery voltage measuring system for precisely measuring the battery voltage is demanded.
Hereinafter, a conventional battery voltage measuring systems will be described. Japanese Laid Open Utility Model Application (JU-A-Heisei 5-47871) describes a battery voltage detecting circuit as a first conventional example. Japanese Laid Open Patent Application (JP-A-Heisei 4-291820) describes an A/D conversion circuit for battery voltage detection as a second conventional example.
FIG. 1 shows a configuration of the battery voltage detecting circuit 110 of the first conventional example. The battery voltage detecting circuit 110 has a power source 101, a battery 105 and an analog/digital (A/D) converter 106. The A/D converter 106 has a reference top terminal to which a higher reference voltage VRT is supplied, a reference bottom terminal to which a lower reference voltage VRB which is lower than the higher reference voltage VRT is supplied, an input terminal to which an analog input voltage VIN between the higher reference voltage VRT and the lower reference voltage VRB is supplied, and output terminals. The reference bottom terminal (VRB) is grounded. In other words, the lower reference voltage VRB is 0[V]. The A/D converter 106 generates a digital output value Dout1 as a result of comparison of a difference between a value of the input voltage VIN and the lower reference voltage VRB and a difference between a value of the higher reference voltage VRT and the lower reference voltage VRB and outputs the digital output value Dout1 through the output terminals thereof.
An anode of the battery 105 is connected to the reference top terminal (VRT) of the A/D converter 106, and a cathode of the battery 105 is grounded. Thus, a battery voltage Vbat is supplied from the battery 105 to the reference top terminal of the A/D converter 106 as the higher reference voltage VRT. An anode of the power source 101 is connected to the analog input terminal (VIN) of the A/D converter 106, and the cathode of the power source 101 is grounded. Thus, a reference voltage Vref1 is supplied to the input terminal VIN of the A/D converter 106 as the input voltage VIN. The A/D converter 106 generates the digital output value Dout1 as a result of comparison between a value of the battery voltage Vbat and the reference voltage Vref1 and outputs the digital output value Dout1. In this manner, the battery voltage detecting circuit 110 in the first conventional example detects the battery voltage Vbat by the digital output value Dout1 outputted from the A/D converter 106. A measurement range is limited to the state Vbat≧Vref1.
FIG. 2 shows a relationship between the battery voltage Vbat and the digital output value Dout1 in the battery voltage detecting circuit 110 of the first conventional example. Provided that resolution of the A/D converter 106 is of n bits, the digital output value Dout1 of the A/D converter 106 in the battery voltage detecting circuit 110 is obtained according to the following equation (1).
                              D                      out            ⁢                                                  ⁢            1                          =                                            V                              ref                ⁢                                                                  ⁢                1                                                                                      V                  bat                                -                0                                                              2                  n                                -                1                                              =                                    (                                                2                  n                                -                1                            )                        ⁢                                          V                                  ref                  ⁢                                                                          ⁢                  1                                                            V                bat                                                                        (        1        )            Here, the resolution is n and the reference voltage Vref1 are both constant, and the equation (1) becomes a linear fractional function having only the battery voltage Vbat as a variable. As shown in the equation (1), when the battery 105 discharges electricity (the portable device is driven by the battery 105), the digital output value Dout1 of the A/D converter 106 increases as the battery voltage Vbat decreases. As shown in the equation (1) and FIG. 2, as battery voltage Vbat decreases, an increase amount in digital output value Dout1 to a decrease amount in the battery voltage Vbat is larger.
FIG. 3 shows a configuration of the battery voltage detecting circuit 120 according to a second conventional example. The battery voltage detecting circuit 120 has power sources 102 and 103, the battery 105, the A/D converter 106 and a voltage divider 107.
The A/D converter 106 has the reference top terminal VRT to which the higher reference voltage VRT is supplied, the reference bottom terminal VRB to which the lower reference voltage VRB lower than the higher reference voltage VRT is supplied, the analog input terminal VIN to which the input voltage VIN between the higher reference voltage VRT and the lower reference voltage VRB is supplied, and the output terminals. The A/D converter 106 generates a digital output value Dout2 as a result of comparison of a difference between the input voltage VIN and the lower reference voltage VRB (Vref3) and a difference between the higher reference voltage VRT (Vref2) and the lower reference voltage VRB (Vref3) and outputs the digital output value Dout2 from the output terminals.
The voltage divider 107 has an input terminal, an output terminal and a ground terminal. The anode of the battery 105 is connected to the input terminal of the voltage divider 107, and the cathode thereof is grounded. Thus, the battery voltage Vbat is supplied to the voltage divider 107. The output terminal of the voltage divider 107 is connected to the analog input terminal (VIN) of the A/D converter 106. The voltage divider 107 generates a divided battery voltage xVbat obtained by dividing the battery voltage Vbat in a voltage division ratio x (0<x≦1) and supplies the divided battery voltage xVbat as the input voltage VIN to the A/D converter 106. The anode of the power source 102 is connected to the reference top terminal (VRT) of the A/D converter 106, and the cathode thereof is grounded. Thus, a reference voltage Vref2 as the higher reference voltage VRT is supplied to the A/D converter 106. The anode of the power source 103 is connected to the reference bottom terminal (VRB) of the A/D converter 106, and the cathode thereof is grounded. Thus, a reference voltage Vref3 as the lower reference voltage VRB is supplied to the A/D converter 106. In this case, the A/D converter 106 generates the digital output value Dout2 based on the divided battery voltage xVbat as a result of comparison of a difference between the divided battery voltage xVbat and the reference voltage Vref3 and a difference between the reference voltage Vref2 and the reference voltage Vref3 and outputs the digital output value Dout2.
In this manner, the battery voltage detecting circuit 120 according to the second conventional example detects the battery voltage Vbat from the digital output value Dout2 outputted from the A/D converter 106. The detection range is limited to a range of Vref3≦xVbat≦Vref2 or Vref3/x≦Vbat≦Vref2/x.
FIG. 4 shows a relation of the battery voltage Vbat and the digital output value Dout2 in the battery voltage detecting circuit 120 according to the second conventional example. Provided that resolution of the A/D converter 106 is of n bits, the digital output value Dout2 of the A/D converter 106 in the battery voltage detecting circuit 120 is obtained according to the following equation (2).
                              D                      out            ⁢                                                  ⁢            2                          =                                                            xV                bat                            -                              V                                  ref                  ⁢                                                                          ⁢                  3                                                                                                      V                                      ref                    ⁢                                                                                  ⁢                    2                                                  -                                  V                                      ref                    ⁢                                                                                  ⁢                    3                                                                                                2                  n                                -                1                                              =                                    (                                                2                  n                                -                1                            )                        ⁢                                                            xV                  bat                                -                                  V                                      ref                    ⁢                                                                                  ⁢                    3                                                                                                V                                      ref                    ⁢                                                                                  ⁢                    2                                                  -                                  V                                      ref                    ⁢                                                                                  ⁢                    3                                                                                                          (        2        )            Here, the resolution n, the reference voltages Vref2 and Vref3 and the voltage division ratio x are constants, and the equation (2) becomes a linear fractional function having only the battery voltage Vbat as a variable. As shown in the equation (2), when the battery 105 discharges electricity (the portable device is driven by the battery 105), the digital output value Dout2 of the A/D converter 106 decreases as the battery voltage Vbat decreases. As shown in the equation (2) and FIG. 4, a decrease amount in the digital output value Dout2 to a decrease amount in the battery voltage Vbat becomes constant (linear fractional function).
As described above, to stably operate the portable device, it is important to control the battery voltage. Especially, in detection of the battery voltage, a voltage at the time when the battery has been used for long time so that it cannot discharge electricity (discharge end voltage) needs to be acquired. For this reason, it is desired to measure the battery voltage near the discharge end voltage with high accuracy.
However, to measure the battery voltage near the discharge end voltage, the battery voltage detecting circuit 110 according to the first conventional example requires an A/D converter having a high resolution. In this case, scale of circuit becomes large, resulting in an increase in power consumption.
In the battery voltage detecting circuit 120 according to the second conventional example, when the voltage division ratio x is made smaller to measure the battery voltage near the discharge end voltage with high accuracy, the measurement range of the battery voltage Vbat becomes larger. That is, as x in Vref3/x≦Vbat≦Vref2/x is smaller, Vref3/x and Vref2/x become larger. Furthermore, as measurement accuracy is made higher, the measurement range becomes smaller. Moreover, the battery voltage detecting circuit 120 requires the voltage divider 107 in addition to the A/D converter 106.
Further improvement is demanded in the battery voltage detecting circuits 110 and 120.